This section provides background information related to the present disclosure which is not necessarily prior art.
Storage, loading, and offloading of crude oil or other hydrocarbon liquids are crucial issues for the oil production industry. Theses issues decide the facility selection for the oilfield development and make great impacts on the money investment, operational cost, and business interests.
There are two major methods to store crude oil or other hydrocarbon liquids. The first method is called “wet storage” or “water pillow storage.” In this method, the oil and seawater are stored together in a same tank. Because of the density difference between oil and seawater, the oil rises to the top of the tank. While the wet storage is adopted with a loading and offloading system, incoming oil displaces an equal volume of existing seawater to keep constant total volume of oil and seawater in the tank, and vice versa.
The wet storage could be applied with a gravity platform sitting on the seabed or a floating platform with underwater oil storage. If the wet storage is applied with a floating platform, an automatic water-adjusting system for maintaining constant total mass is additionally necessary. Great efforts have been made in floating platforms with underwater oil storage during the development of oilfield in the deep sea, such as SPAR platforms with underwater pontoons to store oil or BOX-SPAR concepts.
The second method is called “dry storage.” In this method, oil are stored with inert gas, which is controlled by an inert gas generating, blanketing, and venting system, in order to prevent any exchange of air between inner and outside environment. While the dry storage is adopted with loading and offloading system, the change of oil mass is compensated by the adjustment of the seawater ballast in a different tank that is not fluidly connected to the tank that holds the oil.
The dry storage could be applied with a gravity platform or a floating platform with oil storage. The floating platform at the water surface usually adopts the dry storage to store oil in the ship, such as a ship-shaped FPSO (Floating Production Storage Offloading Unit), a ship-shaped FSO (Floating Storage Offloading), or a SSP (Sevan Stabilized Platform). The floating platform with underwater oil storage may adopt an improved dry storage. In the improved dry storage method, the solution for maintaining constant total mass is to keep the same mass flow rate of oil and seawater. Again, the tank that holds the oil and the tank that holds seawater are not fluidly connected.
There are only two matured storage methods in the above description: dry storage applied with a floating platform at the water surface and wet storage applied with a gravity platform. Each method has its own advantages and disadvantages.
The dry storage applied with a floating platform at the water surface could be greatly impacted by environmental conditions. For example, FPSO/FSO needs a strong mooring system for preventing large variation in environmental conditions. Besides, the required inert gas generating, blanketing, and venting system for the dry storage could cause pollution of oil gas emission. Furthermore, the tank needs to be designed specially with higher cost, because the pressure within the inert gas tank is lower than the seawater pressure.
The wet storage has four major disadvantages as follows.
First, the direct contact between oil and seawater results in a pollution problem. Second, the density difference between oil and seawater makes the weight of the whole system continuously change during loading and offloading operations through equal volume replacement in the storage. If an effective mass of storage is one hundred thousand tons, the weight difference of the whole system could reach ten thousand tons. Therefore, for being applied with a gravity platform, to increase ballast is required to make sure the storage is fixed on the seabed. For being applied with a floating platform, to maintain constant total mass by automatically adjusting water ballast is necessary.
Third, the wet method can only be used for storing water-insoluble liquid products, such as crude oil, rather than a water-soluble liquid, such as methanol. Fourth, heating oil in the storage is difficult because the interface between oil and water is changing.
Besides, the gravity platform has two further disadvantages as follows.
First, the foundation for the storage applied with a gravity platform is selective. Therefore, some seabed can't be used in the gravity platform development. Second, a gravity platform usually needs to be permanently fixed with solid ballast onto the seabed. Therefore, the gravity platform can't float up, move, and be reused in other oilfields at the end of oil production.
Currently, the most common floating platforms for oil/hydrocarbon production include TLP, SPAR, and sub-submersible (SEMI) platforms. However, they usually can't store oil/hydrocarbon in the platforms. FPSO and SSP usually can't have dry well mouth and be applied with oil drilling facility.